Integral Extraction of Opuntia f icus-indica Peel Bioproducts viaMicrowave-Assisted Hydrodiffusion and HydrodistillationRosaria Ciriminna,† Alexandra Fidalgo,‡ Giuseppe Avellone,§ Carmelo Danzì,∥ Giuseppe Timpanaro,∥

Mattia Locatelli,⊥ Diego Carnaroglio,⊥ Francesco Meneguzzo,# Laura M. Ilharco,*,‡

and Mario Pagliaro*,†

†Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy‡Centro de Química-Física Molecular and IN-Institute of Nanoscience and Nanotechnology, Instituto Superior Tecnico, Universityof Lisboa, Complexo I, Avenida Rovisco Pais 1, 1049-001 Lisboa, Portugal§Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche, Universita degli Studi di Palermo, 90133 Palermo,Italy∥Dipartimento di Agricoltura, Alimentazione e Ambiente, Universita degli Studi di Catania, via Santa Sofia 100, 95123 Catania, Italy⊥Milestone, via Fatebenefratelli 1/5, 24010 Sorisole BG, Italy#Istituto di Biometeorologia, CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino FI, Italy

*S Supporting Information

ABSTRACT: The microwave-assisted hydrodiffusion extraction processaffords high quality pectin and betanin from the peel of red and greenOpuntia f icus-indica fruits under mild conditions. An aqueous mixture ofvalued bioproducts is readily obtained from the fruit peel cell water itself,without any water addition. The integral extract can be used as such toformulate nutraceutical beverages and products or, upon straightforwardseparation, to isolate pectin and betanin devoid of chemical contaminantssuitable as ingredients for products ranging from food and beverage tocosmetic and pharmaceutical sectors. Betanin shows exceptional stabilitythanks to the high amounts of antioxidant polyphenols in the extract. Here,we describe the simple extraction process and present spectroscopiccharacterization of the extracts from red and green Opuntia f icus-indica peel.KEYWORDS: Opuntia, Prickly pear, Biophenol, Betanin, Pectin, Antioxidant, Anti-inflammatory

■ INTRODUCTION

Once economically viable and environmentally friendlyindustrial processes for the extraction of valued bioproductsfrom agriculture and food processing byproducts will bedeveloped, they will become a common practice in all thecountries hosting significant agricultural activities.1 So farmostly disposed of as waste, such byproducts are numerousand generally available in large amount at low or virtually nocost, whereas the economic value of the related byproducts ishigh and generally growing. Examples include waste orangepeel as source of essential oil, polyphenols, and pectin;2 coffeeprocessing byproducts as a source of colorants, biophenols, andpectin;3 and waste tomato peel as a source of amino and fattyacids, biophenols, and carotenoids.4

Ubiquitous in Mexico, Arizona, and other sunny U.S. States;in many parts of Central American, North African, and MiddleEast countries; and widely cultivated in Brazil, South Africa,and Argentina, cactus pear Opuntia f icus-indica (OFI) is aperennial plant species belonging to the Cactaceae family,whose fruits and stems (cladodes) afford a number ofphytochemicals of significant nutraceutical importance.5

Accommodating up to 90 wt % water in the cladodes,6 thecactus pear is a water livestock feed reserve that plays anincreasingly important role to combat desertification in severalarid and semiarid countries.7

The OFI fruits are a source of nutritionally relevantcompounds such as amino acids and essential minerals(especially of calcium, potassium, and magnesium). They arealso rich in antioxidant vitamin C (20−40 mg/100 g)8 andsterols,9 as well as anti-inflammatory betalain pigments10

(particularly indicaxanthines)11 and biophenols, impartingthem with neuroprotective, antiulcerogenic, and hepatopro-tective properties.12 Even the OFI seed oil is a valuedbioproduct rich in unsaturated fatty acids, exerting unique skinand hair hydrating action, whose antioxidant and anti-inflammatory properties offer further significant potential as afunctional ingredient of nutraceutical and food supplementproducts.13

Received: January 25, 2019Revised: March 6, 2019Published: March 22, 2019

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. 2019, 7, 7884−7891

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The production of waste Opuntia peel is significant, lastingfor several months, with most peel ending either landfilled orused as animal feed, and a smaller fraction feeding anaerobicbiodigestors.14 Being not edible, indeed, the peel is removedfrom the fruit by all companies selling packaged fresh fruits,jams, and fruit juice.Sourcing betalains, and betanin in particular, from Opuntia

fruits in alternative to the current industrial practice to extractthis natural dye from red beetroot (Beta vulgaris L.)15 has beenwidely investigated, in light of significant potential advantages.For instance, betalains were extracted from Opuntia dilleniiunder various technological conditions,16 from Opuntiajoconostle pulp by water/ethanol or water/methanol mixtures,17

or from hydroponic cultivated red-tube spinach (Spinaciaorelacea L.).18 Contrary to red beetroot, which slowly affordsits rizoma betanin content after two years of cultivation, theOpuntia plant gives fruits several times a year. Betanin is avalued violet−red betacyanin, stable at pH between 3.8 and6.8. It is approved as a food additive by the European Union,labeled E162, and is particularly well-suited for use as a naturalcolorant in beverage, confectionary, bakery, dairy, and frozenproducts. Only the potential application in pharmaceuticalsand cosmetics has been questioned because of the poorstability of betanin; thus, a variety of stabilization techniqueshave been proposed and thoroughly reviewed.19

The same search for new sources can be said of pectin,currently mostly obtained from dried lemon peel and, to alesser extent, from apple pomace.20 The first study suggestingOFI peel as a source of pectin to be used as thickening materialgoes back to 1994, when a team in Italy found out that hot-acid-extracted OFI pectin had a galacturonic acid content of64%, and a low degree of methoxylation (10%).21 Scholars inMorocco and in France reported the first structural analysis ofthe pectic material contained in the OFI peel 10 years later.22

Another 10 years later, Rodriguez-Hernandez and co-workersin Mexico,23 relying on conventional hydrolytic extraction inhot acidic water (treatment of the milled peel with 1 wt %ethylenediaminetetraacetic acid for 2 h at 70 °C and pH 4.0),reported that pectin extracted from the peel of Opuntiaalbicarpa Scheinvar “Reyna” fruits has a low methoxyl content(30.7%), and high molecular weight (Mw = 10.16 × 105 gmol−1 versus Mw = 76 × 103 g mol−1 of lemon pectin), whichimparts the OFI pectin the ability to form soft and elastic gels.We now report the first outcomes of four separate tests

aimed to extract pectin and red dye from the peel of twodifferent types of OFI fruits harvested in Sicily, Italy, viamicrowave-assisted hydrodistillation and solvent-free micro-wave-assisted hydrodiffusion. The latter process, combiningmicrowave (MW) heating and Earth gravity at atmosphericpressure, has rapidly emerged as the most attractive techniqueto extract and to separate value-added bioproducts from solidsamples with various advantages, which include highreproducibility, less energy consumption, shorter procedures,higher purity of the final product, elimination of organicsolvent, and consequent elimination of waste effluents.24

■ EXPERIMENTAL SECTIONSicily’s OFI fruits were used in all the tests. The initial samples ofgreen and red OFI fruit weighed 8 kg each (Figure 1). Twoextractions were performed for each sample, using only the peel of theOFI fruits, and thus releasing the valued edible pulp. Furthermore, thepeel has been milled to enhance the contact surface area during theMW-assisted extraction.

The tests were carried out on a 1−2 kg scale using an Ethos X(Milestone) commercial extractor, as this instrument simulates exactlythe process occurring in the MAC-75 (Milestone), namely, a semi-industrial multimode microwave extractor able to process 30 kg ofbiological matrix per hour, containing a removable, rotating drum thatallows the loading and processing of up to 75 L of biologicalmaterial.25 The rotation ensures a homogeneous microwavedistribution to the material inside the drum.

In each test, a weight (w) of OFI peel was milled and water added(only in the hydrodistillation process). The extraction program usedmicrowave generated by a magnetron at an appropriate power for atime period. The extraction temperature rapidly reached 70 °C, anddid not further change. A summary of the conditions used and theliquid amounts recovered is given in Table 1.

At the end of the procedure, all the solution contained in theextraction vessel was recovered (Figure 2) and made available forpectin, dye, and sugar separation and analysis.

Photographs of the aqueous extracts are provided in Figure 3.The aqueous extracts as such were characterized by infrared

spectroscopy in diffuse reflectance (DRIFT) mode. Pectin wasremoved by precipitation with ethanol at −18 °C (ethanol:watermolar ratio of 25:1), followed by centrifugation at 4000 rpm for 40min. The pectic polymer was readily separated via dialysis andlyophilized, and the powder was characterized by DRIFT. The pectin-free extracts were analyzed by UV−vis spectroscopy immediately afterpectin removal and again after aging for 4 months in the initial vessels,without any additives. The aged extracts were also analyzed byinfrared spectroscopy in attenuated total reflection (ATR) mode.

The DRIFT spectra of the extracts were recorded on a MattsonResearch Series 1 FTIR spectrometer using a Specac selector, with 4cm−1 resolution and a wide band MCT detector (400−4000 cm−1).They were the result of 500 single-beam scans for the sample (groundwith KBr) made as a ratio of 500 single-beam scans for ground KBr,taken as the background. Those of the pectin powder were recordedon a Bruker V70 FTIR spectrometer with 2 cm−1 resolution.

The ATR spectra were also recorded on a Mattson RS1 FTIRspectrophotometer, using a horizontal accessory from Pike Tech-nologies, with a 10 reflections ZnSe crystal. The spectra were scannedwith 2 cm−1 resolution and were the result of making a ratio of thesingle-beam spectrum of the aqueous sample to that of a thin film ofdistilled water (100 scans each), used as background. The UV−visspectra were taken on a double-beam JASCO V-650 spectrometer inthe 350−700 nm region, using 10 mm quartz cells. The baseline wasscanned with distilled water, which was further used as reference.

The analyzed solutions are identified in Table 2, according to theirsource and extraction procedure.

For future commercialization, the water-soluble betalain dyemixture may be processed as such or converted into a stable powdervia lyophilization.

■ RESULTS AND DISCUSSIONThe fingerprint region (1800−900 cm−1) of the DRIFTspectra of the initial aqueous extracts (Figure 4) are better

Figure 1. OFI red fruits (left) and mixed red and green fruits (right).All fruits from Sicily, treated fresh as received.

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resolved when the process is hydrodiffusion-assisted, andespecially when the source is red Opuntia peel. Under thesolvent-free extraction conditions the known efficacy ofmicrowave-assisted hydrodiffusion extraction process affords,for both white and red Opuntia peels, a red aqueous solutionweighing approximately between 32% and 35% of the originalpeel weight (Table 1).There are strong contributions from the pectin bands,26 as

shown in the assignments summarized in Table 3. The strongand broad band in the 1800−1500 cm−1 region is assigned tothe stretching mode of carboxylate groups (νasCOO−),overlapped with bands from carbonyl groups (mostly fromesterified galacturonic acid, ν(CO)ester, and nonesterifiedacidic carbonyl groups, ν(CO)acid), and also with the water

deformation band. A component of the ν(C···C) aromaticmodes of betanin may also contribute to this band.The band at ∼1456 cm−1, visible in all the spectra, is

assigned to the antisymmetric methyl deformation mode,δas(CH3), of ester methyl groups in the galacturonic rings, andof rhamnose rings of the pectin backbone, overlapped withanother ν(C···C) mode of betanin. The carboxylate νsCOO

−,ester ν(C−O−C), and different νC−O modes appear at1410−1417, 1270−1280, and 1234−1240 cm−1, respectively.In the extracts by hydrodistillation the last two bands are muchoverlapped. In the spectra of red Opuntia extracts, a small bandat ∼1313 cm−1 is observed that may be assigned to the δ(C−O−H) mode of pyranose rings or of phenolic rings ofpolyphenols.The group of three intense and partially overlapped bands

observed at ∼1120, ∼1080, and ∼1040 cm−1 are typical ofpectin, assigned to the C−O−C stretching modes of thepyranose ring and of the glycosidic bond, and to a combinationof the C−OH and C−C stretching modes of pyranose rings.The 950−700 cm−1 region contains the bands related to theexternal deformation vibrations of methyl, methylene, andmethyne groups. The band at ∼920 cm−1 observed for theextracts by hydrodistillation is assigned to the rocking mode ofthe ester methyl group, ρ(CH3).The UV−vis spectra of the fresh pectin-free extracts (not

shown) were comparable to those published for aqueoussolutions of ultrasound-assisted beetroot extracts.27 This factreveals the quality of the present extraction processes, since nochemical additive was added at any step. Such quality wasalready self-evident from the colors of the samples (Figure 3).The UV−vis spectra of the pectin-free solutions, aged for 4months in the absence of additives, are shown in Figure 5.The spectra show that the hydrodiffusion extracts are much

more stable than those obtained by hydrodistillation: thesamples extracted by hydrodiffusion, mainly from red OFI,show two partially overlapped bands at 536 and ∼490 nm,assigned to betanin and betaxanthin, respectively.28 As provenby density functional and time-dependent density functionaltheory (DFT and TDDFT) calculations,29 the HOMO ofbetanin is mostly localized on the benzene ring. The mainLUMO ← HOMO transition observed at ∼540 nm is, thus,essentially a π−π* charge transfer in which electron densityfrom the benzene ring is transferred to the dihydropyridinicring.A very strong phenolic absorption (not shown) was also

observed at 280 nm in the spectra of these extracts. Thepresence of polyphenolic compounds may have a stabilizingeffect, protecting the betalain extract under the simple storageconditions employed: the aqueous extracts were kept at roomtemperature for over 4 months, with visible light easilypenetrating the semitransparent polyethylene bottles. In fact,no change in color or in its intensity has been observed,addressing the betalain chemical instability issue which so farhas limited its widespread commercial utilization.30 In contrast,the hydrodistillation extracts changed color to brown over the

Table 1. Conditions Used in the Different Tests and Amounts Recovered

test source w (kg) procedure water (mL) power (W) period (min) Vextract (mL)

1 green Opuntia 2.4 hydrodistillation 150 1500 60 3102 green Opuntia 1.15 hydrodiffusion 1200 40 3653 red Opuntia 2.1 hydrodistillation 150 1500 60 2654 red Opuntia 1.34 hydrodiffusion 1200 40 490

Figure 2. Ethos X extractor (left) whose vessel was filled with milledOFI peel (middle) affording the aqueous extract (right).

Figure 3. Aqueous extracts from green Opuntia f icus-indica peel (left)and from red Opuntia f icus-indica peel (right). (A) Samples obtainedvia microwave-assisted hydrodistillation. (B) Samples obtained viamicrowave-assisted hydrodiffusion.

Table 2. Identification of the Extracted Products Accordingto Their Source and Extraction Procedure

procedure

sample source hydrodistillation hydrodiffusion

initial aqueous extract green Opuntia HDstG0 HDfG0

red Opuntia HDstR0 HDfR0

pectin-free solution green Opuntia HDstG HDfGred Opuntia HDstR HDfR

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storage period, and betanin was no longer detected by UV−visspectroscopy after 4 months; this may be explained by asequence of reactions that occur in aqueous solution,consisting of the hydrolysis of betanin to betanidin (by lossof the glucose moiety), which undergoes further oxida-tion.31−33

We are reminded here that microwave distillation, with its90−95% shorter extraction times when compared, for example,to conventional hydrodistillation, affords essential oils of betterquality in terms of yield and quantity of pharmacologicallyactive products, especially because of the involvement of longextraction periods which favor the release of oxidative enzymesthat promote degradation.34 The solvent-free MW-assistedhydrodiffusion further improves microwave-assisted extractionby reducing the amount of water, thereby dramaticallyincreasing the concentration of biophenol antioxidants whichprevent degradation of the coextracted natural products.Since solvent-free hydrodiffusion was the most effective

extraction process also in the case of Opuntia f icus-indica peels,

the ATR spectra of the solutions extracted by hydrodiffusionafter separation of pectin are compared in Figure 6 for thefingerprint region (below 1800 cm−1).The spectral pattern is very different from the pectin-

containing extracts, being dominated by the bands at 1550/1544, 1417, and 1044 cm−1, assigned to the νCN/νas(COO−), νs(COO−), and ν(CO) modes, respectively.Although the betanin molecule also contains a pyranose ring,the relative intensities in the 1200−1000 cm−1 region are nolonger characteristic of pectin. In addition, considerable shiftsare observed in the carbonyl and carboxylate bands. Apparentlythese extracts are poor in pectin or even pectin-free.The weak bands at 1644, 1506, and 1457 cm−1 are assigned

to the C···C stretching modes of the aromatic ring of betanin,whose infrared spectrum is well-documented in the literature.35

The band assignments are summarized in Table 4.The pectin powder obtained by lyophilization (from mixed

green and red OFI peel) and its infrared spectrum in diffusereflectance mode are shown in Figure 7.

Figure 4. DRIFT spectra of the initial aqueous extracts by (A) hydrodiffusion and (B) hydrodistillation, normalized to the most intense band.

Table 3. Band Assignments of the DRIFT Spectra (Observed Maxima) of the Initial Extracts

wavenumber (cm−1)

extraction by hydrodiffusion extraction by hydrodistillation

assignment red Opuntia HDfR0 green Opuntia HDfG0 red Opuntia HDstR0 green Opuntia HDstG0

ν(CO)ester 1721sh 1728sh 1724shν(CO)acid 1665sh 1653sh 1656shνasCOO

−/δHOH/ν(C···C)aromatic 1626S 1606 1608 1604broadνasCOO

− 1518sh 1516δasCH3/ν(C···C)aromatic 1456w 1456 1462 1456νsCOO

− 1417m 1414 1410 1416δ(COH)puranose/δ(COH)phenol 1313sh 1311ν(COC)ester 1273sh 1281 1282 1255broadν(CO) 1240m 1234 1230sh 1255broad

1149sh 1153ν(COC)pyranose/ν(COC)glycoside 1124m 1126 1120

1103sh 1103 1103ν(COH)pyranose + ν(CC)pyranose 1080m 1082 1078 1078

1059 10571041m 1043 1039 10451012sh 1014sh 1016sh

ρ(CH3)ester 920 918

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The pectin spectrum shows that it is very pure, and the well-resolved bands in the fingerprint region suggest a high degreeof crystallinity. The spectrum was quantitatively analyzed inthe region between 900 and 1850 cm−1, to estimate themethoxyl content and the relative content in galacturonic acidregions. A deconvolution into Gaussian and Lorentziancomponents was performed, using the procedure alreadypublished.25,36

The degree of esterification (DE) was determined by theratio of band intensities of the esterified carboxyl stretchingcomponents to the total band intensities of the esterified plusnonesterified carboxylate bands. The fraction of galacturonic-acid-rich (HG) regions is proportional to the added areas of

the carbonyl plus carboxylate bands made as a ratio against thetotal areas of carbonyl plus carboxylate plus pyranose bands.This ratio yields just a trend when comparing the differentsamples characterized by the same method, not a true fractionof HG regions. The details of the spectral deconvolution andthe intermediate results are shown in Table S1 in theSupporting Information. The final results were comparedwith those of pectin obtained by different extraction methodsand from other sources (Table 5).The degree of esterification obtained in this work (53%) is

higher than the values published by other authors for OFI peelpectin extracted using hot acid,21 and lies between those forcommercial and hand-cut citrus and grapefruit pectin.26

On the other hand, the relative content in galacturonic acid(HG) regions estimated for pectin from OFI peel is quite lowin comparison to those of citrus or grapefruit pectin and formost commercial ones, using the same evaluation method.26

This confirms that the percentage of HG regions is very muchdependent on the source and also on the extraction procedure.Apparently, milling OFI peel, followed by microwave hydro-diffusion, dialysis, and lyophilization results in pectin withlarger percentages of hairy RG regions that promote theformation of more entangled structures, playing a gel-stabilizing role.

■ CONCLUSIONSExtending our recent studies on microwave-assisted extractionof pectin and essential oils from citrus (lemon, orange, andgrapefruit) fresh peel,36 we have used microwave-assistedhydrodistillation and microwave-assisted hydrodiffusion toobtain highly promising aqueous extracts from the peel ofred and green fresh Opuntia f icus-indica fruits harvested inSicily. Under mild conditions (1 h extraction at 70 °C) bothfruit wastes afford red natural extracts of pronounced stabilityunder ambient conditions.Hydrodiffusion, regardless of the absence of added water,

yields higher amounts of aqueous integral extract. Further-more, after 4 months of storage at room temperature, thehydrodiffusion extract fully retained its original red color,

Figure 5. UV−vis spectra of the pectin-free solutions after a 4 monthstorage period for both green and red fruit peels extracted viamicrowave-assisted hydrodistillation and hydrodiffusion.

Figure 6. ATR spectra of the extracts obtained by hydrodiffusion afterseparation of pectin, normalized to the band at 1044 cm−1.

Table 4. Assignment of the ATR Bands of the Pectin-FreeExtracts by Hydrodiffusion

wavenumber (cm−1)

assignmentextract fromred Opuntia

extract fromgreen Opuntia

ν(CO)carb.acid 1716m 1724m1658sh

ν(C···C) ∼1644w ∼1644w1577sh

νCN 1550Sνas(COO−) 1544S 1544Sν(C···C) 1506wν(C···C) 1457m 1457mνs(COO−) 1417S 1417S

1379mδ(COH)phenol 1311m 1311mν(COC)ester 1280m 1280mνCN/νCO(H)phenol 1246m 1240mν(COC)pyranose/ν(COH)/ν(CC) 1128m 1128m

1084S 1084Sν(CO) 1044S 1044S

1018m

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whereas the hydrodistillation extract turned brown, pointing tothe well-documented betanin hydrolysis (i.e., moleculardegradation). The aqueous extract itself, mostly containingbetalains, pectin, polyphenols, sugars, and enzymes, can beused as such as a value-added nutraceutical product. The pecticpolymer readily separated via dialysis and lyophilized has ahigh degree of crystallinity and high methoxyl content (53%degree of esterification).Ferreira and co-workers have recently emphasized how it

will be very important to establish optimum processingconditions to maximize the stability of betalain natural

pigments and their extraction yields, because of theirapplication as functional ingredients and health promoters innutraceutical, pharmaceutical, and cosmetic products.30 This isexactly what the microwave-assisted hydrodiffusion extractionprocess described in this study achieves under mild conditions,affording high-quality pectin and betanin of endless stability,thanks to the high amounts of antioxidant polyphenols, savingon water to directly obtain an aqueous mixture of valuedbioproducts in the fruit peel cell water itself. Alternative toconventional extraction processes in organic solvents or inwater, the extraction of natural products based on microwaveirradiation of plant material and volumetric heating is now anindustrial reality, though in its early days, offering importanteconomic and environmental benefits.37

Chemat and co-workers have already shown that industri-alization of microwave-based hydrodiffusion processes ispossible, for example, by using an existing large-scalecontinuous microwave suitable for the extraction of 10, 20,100, or 1000 kg of plant material per hour in a batch.38

Driven by the consumer demand for healthy and naturalproducts, the global market of natural food colors (derivedfrom fruits, vegetables, seeds, algae, insects, and minerals) isgrowing at a much faster rate than forecasted. For example, in2016 the market was reported generating $1.31 billion inrevenue in 2015 and was expected to grow at over a 5% annualrate between 2016 and 2021.39 However, the same marketanalyst three years later was reporting that the global marketwas valued at $3.51 billion in 2017.40 Betalains are a smallniche of this market, but their growth potential is verysignificant.30 Though more expensive than synthetic red colormolecules, such as “red aza” or “red paba”, used to increase theappearance of food and make it attractive, betalains have thepotential to transform a public health issue into an opportunityfor enhancing public health, exactly as it would happen whenreplacing synthetic food antioxidants with olive biophenols.41

As nations across the world increasingly uptake bioeconomy asa key asset of their economies,42 these findings are ofsignificant relevance to all countries and regions of the worldwhere Opuntia f icus-indica is harvested. The route is open tofood, pharmaceutical, nutraceutical, beverage, and cosmeticcompanies willing to replace synthetic red colorants and citrus-derived pectin with Opuntia-derived betanin and Opuntia-derived pectin.

Figure 7. Pectin powder from the peel of red/green fruit and DRIFT spectrum in the fingerprint region.

Table 5. Degree of Esterification (DE) and Relative HGContent for Pectin Samples Obtained from OFI Peel (ThisWork and Others), from Citrus Matrices and forCommercial Pectin Samples

pectin source procedureDE(%) HG ∝ to (%) ref

OFI peel microwave-assistedhydrodiffusion

53 23 thiswork

OFI peel hot acid 10 64a 21Opuntia albicarpapeel

hot acidic water 30.7 23

red orange outerskin

microwave-assistedhydrodiffusion

39 43 26

red orange peel microwave-assistedhydrodiffusion

30 58 26

red orange waste microwave-assistedhydrodiffusion

25 48 26

lemon outer skin microwave-assistedhydrodiffusion

24 37 26

lemon peel microwave-assistedhydrodiffusion

34 42 26

lemon waste microwave-assistedhydrodiffusion

40 58 26

grapefruit peel microwave-assistedhydrodiffusion

34 55 26

commercial citruspectin A

68 17 26

commercial citruspectin B

62 44 26

commercial citruspectin C

67 35 26

commercial citruspectin D

86 37 26

aAbsolute value.

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.9b00502.

Table summarizing results obtained by deconvolution ofthe DRIFT spectra of lyophilized pectin in twowavenumber regions (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: lilharco@tecnico.ulisboa.pt.*E-mail: mario.pagliaro@cnr.it.ORCIDLaura M. Ilharco: 0000-0001-6994-1464Mario Pagliaro: 0000-0002-5096-329XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank OP Eurocitrus (Belpasso, Italy) for a gift of Opuntiaf icus-indica fruits used throughout this study.

■ DEDICATIONThis article is dedicated to Prof. Giancarlo Cravotto, Universityof Torino, for all he has made to advance the clean extractionof natural products in the past two decades.

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